digital servo with AM ok?

I just bought a high torque digital servo for my BL Revo but it seems quite slugish and it is causing glitches with my throttle control when i pull left or right.

I heard that digital servos are best run with FM but can be run with AM what do you guys think?

Also, do high torque servos draw too much current for a ESC to handle? Im using a novak hv 4.5 with two 7.4v lipo packs. Will i need to get a ubec or something. You'd think that someone could build a decent bec into ESC instead of getting a seperate one or a rx pack.

cheers fellas